It has come to be pointed out that one of the causes of global warming is the greenhouse effect of CO2, and it has become an urgent task on a global scale to provide countermeasures for CO2 to protect the global environment against global warming. CO2 is generated by a wide range of human activities combusting fossil fuels, and there is an increasing demand for suppressing CO2 emissions. Along with such an increasing demand, researchers have been energetically studying methods for reducing and recovering CO2 in combustion flue gas, and for storing recovered CO2 without releasing it to the air, by bringing combustion flue gas emitted from boilers in contact with aqueous alkanolamine solution or the like, for applications in thermal plants and other power plants that consume a large amount of fossil fuels.
Japanese Patent Application Laid-open No. H5-245339 discloses a conventional method for reducing CO2 (carbon dioxide) and SOX (sulfur oxide) contained in flue gas.
This method includes: removing NOX (nitrogen oxide) contained in flue gas by reducing the NOX; removing SOX contained in the flue gas by bringing the SOX in contact with calcium carbonate contained in slurry; removing CO2 by bringing the flue gas having NOX and SOX thus removed into counter-current contact with an amine-based absorbent (aqueous alkanolamine solution) in an absorber, to cause the absorbent to absorb CO2 contained in the flue gas; and regenerating the absorbent by reducing CO2 in rich solution that has absorbed CO2 to obtain lean solvent in a regenerator, and returning the lean solvent to the absorber. In this method, to prevent depleted materials from accumulating in a system through which the absorbent flows, reclaiming is performed in a reclaimer to heat the absorbent and to cause the depleted materials to be concentrated into sludge, and to reduce the depleted materials contained therein. The depleted materials contain: heat stable salts resulting from degradation of alkanolamine, caused by oxidation due to oxygen contained in the flue gas; heat stable salts resulting from alkanolamine reacting with the remaining NOX or SOX; and solid substances such as soot and dust contained in the flue gas.